Well, it does depend a little bit on the species, such as the neons with the disease outbreak.
I personally love Congo Tetras, Serpae Tetras, or Glowlight Tetras.
They are all super active fish and will fill in a tank nicely with some dense plants.
I know they aren't a tetra, but Cherry Barbs go top to nothing in my opinion, they are EXTREMELY active
If you are looking for tetras go to liveaquaria.com and search up "tetra" they have a lot of different species and their care requirements (though I would look up the species separately because they seem to have a lot of wrong info)